For peak efficiency, lawn mower blades should be resharpened or replaced regularly. Removing a blade retaining nut which has been overtightened or rusted in place can be difficult at best, and even dangerous.
The problem is to lock the blade positively against rotation so the high torque necessary to loosen the nut can be applied with a long wrench. Typically, a block of wood, a brick, or other handy object is placed between the blade and discharge chute. If the block slips, a cut or bruised hand may result. Holding the blade with a gloved hand does not guarantee enough leverage to loosen the nut.
The problem is further compounded by the ground level location of the blade and it is only limitedly visible and accessible through the discharge chute unless it is run up on blocks, elevated on jacks, tilted backwardly, or turned upside down.
As a practical matter, a heavy riding mower is generally not elevated on blocks just to remove the blades for sharpening. Instead, the mower deck is elevated as far as it will go, which will give about 5" of visibility and working space for a person lying or crouching on the ground.
One attempt to solve this problem of holding the blade is described in Taylor Patent No. 4,564,991 issued Jan. 21, 1986 on "TOOL FOR HOLDING A BLADE FOR INSTALLATION AND REMOVAL THEREOF". This involves use of a special tool which works only if the mower is turned upside down; as a result it can be used only with very light weight lawn mowers. However, it is unsafe to turn any mower upside down without first draining the tank, because of the hazard of leaking fuel. The tool shown in that patent engages the mower blade between a pair of stop pins. It could not be used effectively in the limited visibility and accessibility conditions under a riding mower. The tool would fall off because of the open space between the stop pins.